I think it's a debate worth having. Personally I am heavily in the pro-privacy, limited state power camp on most issues. However, if:

(a) the technology is being deployed in specific environments where there really is a credible threat of criminal behaviour, and

(b) it is being used only as an initial filter prior to normal, lawful police action (or inaction) with normal safeguards, and

(c) the target data set used for comparisons is generated on a lawful and justified basis

then I'm not sure there is anything inherently unethical about this sort of technology. Is it really any different to having police officers at the entrances to a stadium with mugshots of known hooligans who have been identified while discussing trouble-making online ahead of the match?

Whether it is technically capable enough to be useful and cost-effective is a different matter, and it's also a test that might give a different result tomorrow.

Whether it is possible to have sufficient safeguards that a system could reliably meet the standards above is also a different question.

And whether the public would trust that those safeguards existed and have confidence that the standards were being met is another question again.